When something is mobile it can move or be moved easily. If you walk to the end of the block talking on your cell phone, you are a mobile being using a mobile device.
A mobile is something you hang over a crib to entertain the baby, or a scale model of the universe. In 1940, the word mobile was first used to refer to a house that could be easily moved — a mobile home. If someone says to you jokingly that they "are not mobile," it means that their car is not working. It will be hard for them to be upwardly mobile, or to improve their social status, if they don't get mobile and get to work soon!